That's a very good point.
That can mean up to 3 things.
1. Chopper was taking the risk of not getting the point he wants while fighting Kuma bot.
2. Chopper has done some off screen stuff to improve upon his rumble ball or concentration or whatever needed to overcome that. (heck, we never found out about the aspect of Usopp's weapon until an arc later)
3. We are underestimating how much time has passed. It seems like only an hour at most has passed since he punched tenryuubito, but could be up to 6 I guess. (we really don't know how long it would take to go from say grove 1 to grove 13. how long they talked ect. Only really basing it off of other rookies fighting, but who knows how far away they parked their boat and how long it took to get there)
Edit: Plus who was planning on leaving. really on sounded like Capone was heading off and maybe Bonny as well. Others seemed to kind of want to stick around and see who's coming and what not. So they may have been sitting around doing nothing for a while. Also Kidd and Law according to last chapter discussion appear to have taken down their bot as well already. so i could see it having already been 6 hours or so.
I'm hoping for #2 since that is a logical action for him to take after almost being killed by Kumadori and shows Chopper's knowledge is growing.
I really don't think six hours have past since he first punched the tenryuubito. A doubt it took long for the admiral to arrive and the entire situation seems too urgent for so much time to pass. As you said, this is an assumption, but I'm guilty of being one of those fans who like to see characters improve significantly, combat-wise.